The Interaction of Wings in Different Flight Modes of a...

17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014

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The Interaction of Wings in Different Flight Modes of a Dragonfly

Csaba Hefler1, Huihe Qiu1,*, Wei Shyy1

1: Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and

Technology, Hong Kong SAR, China * Correspondent author: [email protected]

Abstract Flow fields around the wings of a live dragonfly (Pantala Flavenscens) have been studied using different visualization techniques. Flapping flight and tandem wing configuration are of particular interests for applications on micro air vehicles aimed to achieve similar flight agility as that of a dragonfly. High speed video recordings were used to determine core values of the flapping exhibited by the tethered specimen. These frames were also used to confirm wing alignment on PIV frames. A novel smoke visualization technique has been used to obtain base data for comparison with in phase flapping PIV results. The tethered dragonfly exhibited different flapping patterns which resulted in four distinct flow field structures; however, some were achieved by different flapping modes. In phase flapping is particularly interesting as it is considered a flight mode of the dragonfly when extra thrust are needed for maneuvering or take off. In some cases out of phase flapping generated two separate air streams that supposedly give more stability to the emerging dragonfly. This kind of flow structure has not been comprehensively studied yet. Experiments have been done in still air so the results are not affected by a mainstream flow like in the case of wind tunnel experiments. The general description of the flow fields found is discussed and some points were compared with earlier studies to support the development of tandem wing micro air vehicles. 1. Introduction Micro Aerial Vehicles (MAVs) conceptually limited to the size of less than 15cm, capable of stable flight in urban (often gusty) environments with acceptable payload capability and mission time, are the focus of many academic works (Shyy et al., 2008). Military and civil applications such as reconnaissance or hazardous sites explorations can directly save lives, indicating the importance of these research efforts. Flapping wing locomotion has promising features to be considered in the targeted size region for MAVs. One of the reasons for this is the low Reynolds number effect. In a low Reynolds number region, where MAVs and natural flyers operate, viscous forces and unsteady aerodynamics must be considered as they account for a number of unique phenomena. At such conditions traditional designs are disadvantaged over a flexible flapping wing which can extract additional aerodynamic forces through unique unsteady flow mechanisms (Shyy et al., 2008; Shyy et al., 2010). Bio inspired designs offer a viable solution for MAVs. Among natural flyers, dragonflies have unique features and flight capabilities. They possess 2 pairs of wings which they can move separately; this gives flexibility to adopt flapping kinematics to the environmental conditions. The interaction between the fore and hind wing is a particularly interesting topic. The phase difference highly affects the force production of tandem configuration, and whilst the highest force peaks are achieved when the wings are flapping in phase, the efficiency increases if the hind wing leads with a certain phase difference (Lian et al., 2013). Talking about forward flight or hovering, different authors found different phases to account for the highest efficiency (Wang and Russel, 2007; Sun and Huang, 2007; Usherwood and Lehmann, 2008). Downwash and shed trailing edge vortex (TEV) of the forewing negatively affects force generation of the hind wing negatively as stated in recent works (Sun and Lan, 2004; Maybury and Lehmann, 2004). Downwash inevitably lowers the effective angle of attack of the down stroking hind wing, thus negatively effecting hind wing leading edge vortex (LEV) formation, however shed vortexes if meeting with the hind wing in an optimal setup, can be beneficial for force production of the hind wing, by promoting LEV formation. This positive effect is experimentally studied (Maybury and


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Lehmann, 2004; Rival et al., 2011) however, simplified flapping kinematics were used in the first study, and large wing spacing in the latter, which could result in different results from a real dragonfly. It is in our interest to see whether downwash and vortex shedding is a prominent feature of wing wake interaction of a live dragonfly under near takeoff or hovering conditions. Vortex shedding on the other hand is affected by the forward flight speed and the flapping kinematics of the forewing. Downwash is present during almost the whole course of the hind wing down stroke; however at which instant a shed vortex is interacting with the hind wing can be an important aspect of wing wake interaction. In this study, experiments in still air were conducted, where vortex shedding is not affected by mainstream flow, as in experiments carried out in wind tunnel (Tsuyuki et al., 2006; Thomas et al., 2004). Supposedly, dragonflies can consciously adopt their flapping kinematics for optimal wing-wake interaction according to the global flow conditions. This assumption stands for normal cruising flight, where flight efficiency is of major importance. A recent experimental study (Usherwood and Lehmann, 2008) shows that although interaction is detrimental for lift generation, it has a positive effect on flight energetics. Flow visualization experiments on live dragonflies were performed to confirm whether the flow field shows a similar form for controlled experiments and numerical simulations with those of simplified flapping kinematics and wing shapes. Our visualization experiments also revealed other flow phenomena supposedly characteristic of four winged flight. At flight initiation, sometimes two separate flow streams are present which may improve stability for the dragonfly at takeoff. Global flow conditions of a dragonfly at takeoff, with in phase flapping is not well documented yet; however, it can be an important aspect for the practical design of MAVs. In this paper some finding are also presented related to this flight mode. It is anticipated that these results give useful additional data for the understanding and characterization of tandem wing flapping flight to aid the design of highly agile micro air vehicles. 2. Experimental Setup and Methodology In our experiments the flow fields around dragonflies commonly known as wandering gliders (Pantala Flavenscens) were studied. It is a very common species (not protected) in Hong Kong. Its availability makes it our primary subject; however it is worth mentioning that this species is well known for its supreme gliding skills. This species is a medium sized dragonfly with a wingspan of 80-90 mm, and body length of about 50 mm. The dragonflies were used within a few hours after capture; allowing sufficient time for the specimen to adapt to the lower temperature in the laboratory. The Dragonfly was glued to a transparent glass plate with transparent epoxy glue at its thorax. The 1.1 mm thick glass with narrowing sides towards the dragonfly abdomen was rigid enough to eliminate vibrations and its transparency ensured minimal glare. Our primary tool for measuring the flow field around the flapping dragonfly wings was a particle image velocimetry (PIV) setup (Fig. 1). The flow field measurement was done in a closed air reservoir measuring 1200x800x1000 mm (LxWxH). The distance between the dragonfly and the front wall was 500 mm; side wall was 400 mm; bottom wall was 60 mm. The other walls were farther from the measurement window. The light sheet was set at half span of the right side wing. The flow was seeded by a 4 nozzle aerosol generator of working pressure 6 bar (LaVision, Item number: 1108926). Extra virgin olive oil was used for seeding particle generation. The generated particle sizes averaged approximately 0.3 µm, which proved to reflect enough light for the receiving camera (Kodak Megaplus ES 1.0 TH). After seeding the air reservoir we waited 20 minutes for the flow to be of from disturbances. The system uses a double pulsed Nd:YAG laser of maximum output power 200mJ (Newwave Gemini 200), with a maximum 15 Hz pulse frequency in single frame mode. For our measurements a 532 nm laser with pulse width of 6 ns was used to illuminate the flow field. 80% of the maximum laser power gave adequate light for the measurement. Pulse interval of the two lasers was set to 100 µs in cross–correlation mode. The final interrogation window of 32x32 pixel was used for the cross correlation, with an overlap of 50%. Images were captured using a 1 million pixel camera. The camera has an active area of 9.1 mm*9.2 mm on CCD with pixel size 9 µm*9 µm, resulting in a maximum resolution of 1008*1008


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pixel. It has a triggered double exposure mode which is well suited for PIV experiments because of the short time between successive exposures. The lens used in the experiments is 50 mm focal length Nikkon AFD. One drawback of this setup is that its frequency did not make it possible to sample the air flow at multiple positions of the same flapping cycle as the dragonfly flaps its wings with a frequency of about 30 Hz. The resulting vector field was cleared by setting an adequate correlation peak ratio, and median filtering.

Fig. 1 Pictures of the PIV setup (please note that the dragonfly was placed farther from the front wall in the

actual experiment). The reflected light on the wings caused strong glare which caused errors in the vector calculations, so it was removed with a combined mask on each recorded frame. This results zero velocity around those areas after the cross-correlation. High speed video recording and smoke visualization experiments have been done on live dragonflies to gain supporting data on tethered flapping behavior and flapping kinematics of the dragonfly species. High speed recording with 1000 fps, helped determine the average flapping frequency, amplitude, stroke plane angle and phase difference between the hind wing and forewing. High speed video frames were also used to confirm the position of each wing on the PIV frames when it was questionable. An illustration of this can be seen in Fig. 2. To the best of our knowledge, there was no experimental data available for a live dragonfly flapping in phase in still air. Thus to support our findings smoke visualization experiments were conducted under similar conditions. A dragonfly was vertically tethered in a closed reservoir and a dry ice generated smoke curtain flowed over the wings from a narrow gap on the top of the box. The flow evolution was recorded by high speed camera set for 500 fps. The velocity of the falling smoke was approximately 0.2 m/s which simulated close to hovering conditions for this experiment.


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Fig. 2 Comparison of high speed video image with PIV image for estimating the flapping phase difference;

In this case the forewing is starting its down stroke while the hind wing has passed its mid down stroke phase.

3. Results and Discussion The research aim at this stage is to give a general overview of the flow field around the dragonfly wings for different flapping modes, to be extended by flapping kinematics measurement for the basis of a more complete numerical simulation in follow up studies. The PIV setup was capable of taking randomly timed snapshots of the flapping dragonfly. The experimental results are categorized according to forewing and hind wing phasing as in phase flapping and out of phase flapping. In-phase flapping generates higher lift forces required for takeoff (Alexander, 1984), so it is assumed that those frames are picturing the flow field of the first few flapping cycles of a dragonfly trying to take off. Out of phase flapping is energetically less demanding (Alexander, 1984; Maybury and Lehmann, 2004; Usherwood and Lehmann, 2008), and often occurs for emerging and forward flight. This categorization is too general considering the flight capabilities of a dragonfly, and the flow structures of our results. Accordingly our results were also categorized according to the flow orientation generated by the flapping wings. If the stream is vertical or close to vertical we refer to that as vertical takeoff flight mode; if it is aligned approximately 45o to the horizontal plane we refer to that as emerging flight mode, and if the stream is nearly horizontal we refer to that as forward flight mode. It is worth pointing out that the dragonflies in our experiments were capable of generating these different flow orientations with in phase flapping as well as out of phase flapping. According to the high speed recording of the tethered dragonfly the forewing and hind wing phase difference continuously changes through the recorded cycles (between 80-110 degrees), with hind wing leading. The values for the out of phase flapping on PIV images were in the same range. The flapping frequency is averaging 29.5 Hz through the recorded cycles (Fig. 3 shows an example of this) and is expected to be close to the values the dragonfly exhibits through the PIV measurements. The flapping amplitude is 60.9o

for the forewing and 53.74o for the hind wing on average (only full strokes considered). Note that these values are calculated not from the wingtips but at the endpoints of the pterostigma on the wings that could be easily identified on the


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individual frames. The stroke plane angle for the forewing was between 40-50o and 40-60o for the hind wing using the body horizontal axis as reference. Reynolds number according to the forewing chord at mid span (9.4mm), and wingtip velocity is in the range of 1800-2000. Similar values were observed on the PIV frames. These values are close to the values found for other dragonfly species (Azuma and Watanabe, 1988; Wakeling and Ellington, 1997).

Fig. 3 Stroke angle time histories extracted from high speed video recording. Angle is given relative to the

wing root, which in this setup is on the same horizontal line for both wings. (Note: the uncertainties near the 4th and 5th hind wing down stroke are caused by frames where the position of the pterostigma couldn’t be

identified clearly). Smoke visualization experiments showed the presence of a forewing leading edge vortex and a hind wing trailing edge starting vortex for a first down stroke with in phase flapping (Fig. 4). A hind wing LEV was also apparent, however, only for the first down stroke. The TEV shed with a direction along a line of angle 45o from the body axis at the border of the generated stream. The LEV on the forewing and hind wing stayed attached until the start of the supination. The second down stroke, also in phase flapping, showed similar phenomena but the shedding of the TEV changed direction; it shed along the body axis of the dragonfly. No hind wing LEV was apparent for the second down stroke. The generated airflow was directed downwards with approximately 45o alignment to the body axis. The flow direction and the vortex positions were in agreement of those apparent on the PIV measurements for in phase flapping. One of the most interesting phenomena of our experiments was a flow field that consisted of two separated airflows one oriented downwards while the other was more towards the body axis of the dragonfly (Fig. 5). According to our best knowledge this phenomena have not previously been recorded. There are two hypotheses explaining this unusual phenomenon. First the dragonfly can flap their wings individually, with different angular velocity and stroke kinematics. There is a possibility that the two separate streams originate from the forewing and the hind wing respectively. This could be a convenient explanation for streams, not parallel (Fig. 5). Another possibility is that the hind wing actually separates the flow generated by the forewing. This seems reasonable for parallel running flow channels (Fig. 6). Very similar hind wing positioning on both frames seems to support this observation. This can be another example of how the hind wing interacts with the forewing wake, for a more stable emerging flight. Fig. 6 shows two frames which are from separate flapping cycles, but show close similarities as if they would be captured from the same cycle with a short time difference. It is worth mentioning that this phenomenon was found only for out of phase flapping.


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Fig. 4 Vortex development and shedding at the first down stroke of an in phase flapping dragonfly.

Forward flight flow field can be seen in (Fig. 7). The forewing is in its upstroke whilst the hind wing is at its upmost position. Horizontally directed flow was found only in the case of out of phase flapping. Clearly the wake of the forewing reaches the hind wing upper surface at an acute angle which generates negative lift. Negative vertical force for the up stroking hind wing in the case of forward flight was also found for a number of tested cases in recent studies (Wang and Sun, 2005; Broering and Lian, 2012). Forewing vortexes are not apparent as expected for the upstroke, when the wing plane is oriented almost vertically to reduce drag.


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Fig. 5 Out of phase flapping generates two separate flow streams (not parallel). The forewing has passed its

mid upstroke, while the hind wing is at its upmost position. In phase flapping generated downwards oriented flow (Fig. 8) in half of the cases and 45o downwards oriented flow (Fig. 9) in the other half of the eight identified cases. However there is a difference of 45o between these directions that is achieved with a seemingly small change in the stroke plane angle. At this point it seems that the dragonfly has the means to conveniently direct the generated flow and thus the resulting lift and thrust forces between these two extremes; however it is not proven if in phase flapping would be used to generate horizontal flow for thrust enhancement in the forward direction. In the case of in phase flapping, the flow field is apparently smooth and uniform, suggesting that the two wings function almost as a single wing, without any major wing wake interaction. Out of phase flapping is considered the preferred flight mode for normal cruising and moderate maneuvering moves dragonflies (Alexander, 1984). Besides, the special cases mentioned earlier with out of phase flapping, the dragonfly generated 45o directed flow in a majority of the recorded cases (emerging flight– Fig. 10) with only one case where the dragonfly used this mode for vertical takeoff (Fig 11). Hind wing TEV and forewing LEV formation were observed in most of the recorded cases. The position of the shed TEV was above the top side of the hind wing, and in every recorded case only one could be identified. Forewing TEV could not be identified clearly on any of the recorded cases, neither do forewing shed LEV appears near to the hind wing. It seems that forewing wake and hind wing interaction for these conditions is limited to the downwash effect (detrimental for lift generation (Sun and Lan, 2004; Maybury and Lehmann, 2004)). Results suggest that out of phase flapping can be generally used by the dragonfly for takeoff, cruising or emerging flight. However the number of occurrences shows that it is not the preferred mode for vertical takeoff.


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Fig. 6 Two frames showing out of phase flapping to generate separate flow streams (parallel). A starting TEV

above the hind wing and signs of forewing LEV are also visible on these frames. The forewing is in its mid down stroke position in the first frame and at the end of its down stroke in the second frame, while the hind

wing is starting its upstroke in the first frame and around at mid upstroke in the second frame.


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Fig. 7 Out of phase flapping in forward flight mode where a shed upstroke TEV of the hind wing can be seen

at the lower part of the wake.

Fig. 8 In phase flapping generates a vertically directed flow stream where a very definite hind wing TEV and

signs of a forewing LEV are also observable.


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Fig. 9 In phase flapping generates 45o directed flow stream where signs of forewing LEV are also observable.

Fig. 10 Out of phase flapping generates 45o directed flow stream where a very definite hind wing TEV is also observable. Here the forewing is near at the end of its down stroke, while the hind wing supinates before the

start of up stroking. 4. Conclusion Experiments were conducted using high speed visualization and PIV techniques on tethered, live dragonflies for measuring the flow fields around their flapping wings. The experimental conditions were close to dragonflies in takeoff and hovering conditions. Therefore, our experiments were not overly dominated by the free stream velocity often seen in wind tunnel


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experiments.

Fig. 11 Out of phase flapping generates a vertical flow stream. Here the forewing is starting its down stroke

while the hind wing has passed its mid down stroke phase. Vertically oriented and well focused flow stream were observed by the flapping dragonfly under both inphase and out of phase flapping (Fig 8 and 11). This suggests that vertical take off as well as howering in still air can be exectuted by both wing phasing as the dragonfly body is horizontally positioned and the stroke plane angle is backwards tilted. Quantitative measurements of the flow field of the hovering dragonfly for 90 degrees out of phase (hind wing leads) and in phase flapping exhibited by a dragonfly hovering with horizontal body alignment and tilted stroke plane angles were conducted. Experimental results by using live dragonfly flapping in still air to quantitatively describe the flow field of a hovering dragonfly has not been done according to our best knowledge. The wake in our measurements are free from shed vortexes (fig 8 and 11), which is different from that of reported by Wang and Russel (2007), despite of the higher Reynolds number exhibited by a live dragonfly. This suggest that fine adjustment of the flapping kinematics executed by the live specimen help to extract additional force and produce a fine wake structure without excess swirl. It is found that in phase flapping can generate a well-focused stream, directed vertically or at an angle of 45o. By the number of occurrences (90% of the identified cases) we can conclude that it is the preferred mode to generate downward momentum for vertical takeoff. In case of in phase flapping, the two wings seemingly functions as one large wing, which apparently generates one distinct TEV starting vortex and one LEV on the forewing. Seemingly there is no interaction of the wings in this case. Another observation in the experiments is that a dragonfly is capable of generating two separate flow streams, in an out of phase flapping mode. This supposedly adds extra stability for the dragonfly when it changes from taking off to forward flight. Acknowledgements


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The work described in this paper was supported by the Hong Kong University of Science & Technology and Hong Kong Ph.D. Fellowship Scheme from the Research Grants Council (RGC) of the Hong Kong Special Administration Region, China.

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